The present invention is directed to a completely-in-canal (CIC) hearing aid constructed to have a robust feedback stability.
A typical CIC instrument can operate normally when its acoustic gain does not exceed 40 dB. If the acoustic gain of a CIC instrument gets increased, the frequency response of the instrument develops sharp peaks. After the gain of a CIC instrument exceeds a certain threshold, the instrument becomes unstable and begins to oscillate at a frequency of the highest peak of the frequency response. A typical CIC instrument comprises a shell, a faceplate, a battery, a hybrid, a microphone, and a receiver.
The purpose of the receiver is to convert the electrical signals into an acoustic sound pressure. As a by-product of its operation, a receiver creates mechanical vibrations. A typical CIC receiver is shown on FIG. 1. Its construction is described in related art publications (e.g., U.S. Pat. No. 6,078,677), in which its FIG. 2 (corresponding to FIG. 1 in the present application), shows a longitudinal section of an electroacoustic transducer 1.100, wherein an actuator comprises an electric coil 1.31 which is connected via an electric line 1.32 extending through the lower case 1.4 to terminals 1.33 mounted on the outer surface of the housing 1.2. Placed within a magnet housing 1.34 is a magnetic member 1.35. An air gap 1.36 of the magnetic member 1.35 is aligned with an air gap 1.37 of the coil 1.31. A U-shaped armature 1.40 has a first leg 1.41 attached to the magnet housing 1.34 and a second leg 1.42 extending into the aligned air gaps 1.36 and 1.37. Attached to the end of the second leg 1.42 is the fork 1.21.
If an externally generated current is presented to the coil 1.31, a force is exerted on the armature 1.40 by the magnetic field generated by the magnetic member 1.35. As a result thereof, a displacement is generated in the longitudinal direction of the fork, thereby moving the diaphragm to generate a pressure wave. The cover 1.3 has an opening 1.46 through which the interior of the housing 1.2 between the cover 1.3 and the diaphragm 1.10 communicates with the outside world. Attached to the housing is a substantially cylindrical nozzle 1.47 to which, if desired, a flexible tube can be fastened for guiding pressure waves.
This figure shows that the diaphragm 1.10 may have a layered structure. More in particular, the diaphragm 1.10 comprises a thin flexible foil 1.51 and a reinforcement layer 1.52 attached thereto, e.g. by gluing. The reinforcement layer 1.52 has a thickness exceeding that of the foil 1.51 and has a surface defining the central diaphragm portion 1.11. The part of the foil 1.51 projecting beyond the reinforcement layer 1.52 defines the edge portion 1.12.
A simplified vibration model of a CIC receiver is shown in FIG. 2 of the present application. It comprises a case 20, a U-shaped armature 30 and a membrane 40. The motor (not shown for simplicity of illustration) creates various forces 50 that cause the membrane 40 to vibrate: the force applied to the U-shaped armature 30 and the reaction force applied to the case 20. The major reason for an unstable operation of a CIC instrument is the receiver vibrations that cause the CIC faceplate to radiate the sound pressure into the microphone.
Therefore the receiver has to be isolated from direct contact with the shell or other CIC components inside a CIC instrument. As FIGS. 3 and 4 illustrate, the receiver 100 of a typical CIC instrument is placed inside the CIC shell 12 and attached to the shell tip 14 with a flexible tube 66. The tube 66 feeds the sound pressure, generated by the receiver 100, into the ear of the user. The tube 66 also isolates the vibrations that the receiver 100 creates, from spreading into the CIC instrument. A receiver in a conventional CIC instrument also has a soft boot or studs 60 that prevents the receiver walls from forming a direct contact with CIC elements inside the shell 12. When such a direct contact occurs, the CIC becomes unstable or its frequency response begins to comprise many sharp and undesirable peaks.
A conventional CIC instrument performance is not consistent. Even if the same assembly worker builds two “identical” instruments, their performance would be quite different because the position of a receiver inside the shell is not fixed. The receiver can be moved and turned before it is fixed to its final position, and the worker can not replicate such a position even if the shell of the second instrument is exactly the same as that of the first instrument.
A construction of a receiver and a hearing instrument is described in the related art U.S. Patent Publication No. 2005/0074138; this reference teaches how to build instruments with a higher consistency of performance. In the instrument disclosed, the virtual receiver position is chosen by using custom 3-D software before the shell is manufactured. Then, the real shell is produced by a stereo lithographic apparatus (SLA) process with all necessary features that will support the receiver in a designated place. The construction of the receiver and the supporting structures guarantee that the CIC instrument will operate without the feedback with the acoustic gain up to 40 dB.
A typical construction of a CIC instrument 10 with an RSA receiver 100 is shown in FIG. 5, that comprises a faceplate 11, shell 12, microphone 13, battery door 15 with battery 16, hybrid 17, and vent 18. Such an instrument has a maximum gain limitation of 40 dB. However, a CIC instrument with an RSA receiver can be built with higher maximum stable gain by following special rules during its virtual assembly.